1. Field of the Invention
This invention relates to a four-quadrant converter which transfers power between a direct current and an alternating current system. More particularly, this invention discloses a four-quadrant full-wave static converter providing quick response without current cross-over deadband delay.
2. Description of the Prior Art
A direct current motor may be operated in four possible modes: (1) as a motor with a clockwise rotation; (2) as a motor with counterclockwise rotation; (3) as a generator with clockwise rotation; and (4) as a generator with counterclockwise rotation. These modes of operation are referred to as quadrants. The four quadrants can be represented on a rectangular coordinate system with speed being indicated on the vertical axis and current being indicated on the horizontal axis so that clockwise motoring, with positive current and positive speed, is represented by quadrant 1, clockwise generating, with negative current and positive speed, is represented by quadrant 2, counterclockwise motoring, with negative current and negative speed, is represented by quadrant 3, and counterclockwise generating, with positive current and negative speed, is represented by quadrant 4.
A converter converts AC power to DC power to supply the load or when the load is acting as a generator converts DC power back from the motor and applies it to the AC lines. When the converter is changing DC generator power to AC line power, the converter is said to be inverting. Halfwave converters create DC current having a poor form factor and causing a DC component on an AC line which can interfere with circuit components are usually not desirable for supplying a rotating direct current load. A basic single-phase full-wave power converter utilizes four thyristors connected in a full-wave bridge rectifying configuration. A basic single-phase two-quadrant full-wave converter of this variety is capable of operation in quadrants 1 and 4 or, alternatively, in quadrants 2 and 3.
A converter of this type would be useful in applications such as a hoist. The converter would supply DC power to the hoist motor to lift the load vertically. This would be in the motoring mode, or quadrant 1. As the load is lowered, the motor acts as a generator while being rotated in the opposite direction feeding power back into the alternating current line.
For many operations, a four-quadrant single-phase full-wave power converter is necessary. Four-quadrant converters normally consist of a pair of two-quadrant converters interconnected in such a way that one group can conduct positive motor current and the other group can conduct negative motor current, thus providing for operation in four quadrants. For example, one simple thyristor group allows converter operation in quadrants 1 and 4 and the other group allows converter operation in quadrants 2 and 3. The four-quadrant converter is sometimes referred to as a back-to-back converter since the operation of each full-wave thyristor group has historically been considered independent. Note that operation in quadrants 1 and 4 is no different than operation in quadrants 2 and 3 other than the direction of current flow with respect to the DC load. A major problem which can occur with prior art four-quadrant converters occurs at the instant of changing polarity of the current. Once a thyristor has been turned on, it remains a short circuit until the current flow through it is reduced to 0. If selected thyristors in both groups are conducting simultaneously, a short circuit can occur across the affected thyristors. This type of failure is termed an "AC line shoot-through". This failure would cause a catastrophic failure of the power converter and is usually prevented by modifying the way the thyristors can be triggered. The drivers for the thyristors are modified to include a deadband region. This deadband region prevents rapid change in the current polarity.
Also, most prior art four-quadrants converters utilize some means of determining when the armature current is 0 and then waiting a predetermined time before allowing triggering of thyristors which conduct current of a different polarity. This usually involves expensive and complicated circuitry which provides a deadband region. The deadband region is made large enough so that when the signal calling for change in polarity of the armature current goes through 0, all thyristors remain off so that there can be assurance that both groups will not be firing or conducting simultaneously.
It is generally known that in continuous conduction the output voltage of a phase controlled static converter is a function only of the firing angle and AC line voltage. It is also known that in discontinuous conduction the output voltage is also a function of the load impedance and countervoltage. In continuous conduction the output voltage can be made to vary almost linearly with respect to the driver input voltage. When the discontinuous conduction region is reached the transfer function of output voltage versus driver input voltage takes on a new slope dependent on load impedance and countervoltage. This change in driver-rectifier gain which occurs when crossing between the continuous and discontinuous conduction regions presents difficulties in maintaining maximum response characteristics and stabilizing any closed loop system, which utilizes the driver-rectifier gain. Still generally the compensation which is used to stabilize the loop during discontinuous conduction limits the bandwidth which can be achieved in continuous conduction.
It is generally known that the effects of this gain change can be greatly reduced by creating a current overlap at zero motor current with back-to-back converters.
Historically, this has been achieved by utilizing back-to-back halfwave converters and basing both on slightly around the zero current area. The result is alternate polarity current pulses which add to a net zero DC value. Thus a finite conduction angle is achieved at zero current and the driver-rectifier characteristic is effectively linearized by the AC bias at low DC currents where extreme discontinuous conduction would otherwise occur. This same linearizing of the driver-rectifier transfer is achieved with the invention described but with the further advantages of full-wave conversion at higher current levels and reduced susceptibility to noise induced shoot-through which could cause failure of the converter.